#! /bin/sh
#
# resolv.conf merge hook for rdnssd

# *************************************************************************
# *  Copyright © 2007-2009, 2017 Pierre Ynard.                            *
# *  This program is free software: you can redistribute and/or modify    *
# *  it under the terms of the GNU General Public License as published by *
# *  the Free Software Foundation, versions 2 or 3 of the license.        *
# *                                                                       *
# *  This program is distributed in the hope that it will be useful,      *
# *  but WITHOUT ANY WARRANTY; without even the implied warranty of       *
# *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        *
# *  GNU General Public License for more details.                         *
# *                                                                       *
# *  You should have received a copy of the GNU General Public License    *
# *  along with this program. If not, see <http://www.gnu.org/licenses/>. *
# *************************************************************************

set -e

PATH=/sbin:/bin

# Max number of nameserver options taken into account. Should be as
# defined in <resolv.h>
MAXNS=3

# This script tries to share available nameserver slots with IPv4
# entries, for example to allow fallback to IPv4 if IPv6 fails. If
# there is not enough room for all IPv6 and IPv4 entries, this script
# will limit the IPv6 entries it adds to $RDNSS_LIMIT only.
RDNSS_LIMIT=$(($MAXNS - 1))

sysconfdir='@SYSCONFDIR@'
localstatedir='@LOCALSTATEDIR@'
resolvconf="$sysconfdir/resolv.conf"
myresolvconf="$localstatedir/run/rdnssd/resolv.conf"

# These should be POSIX-compliant BREs
RE_NSV4='^nameserver  *\([0-9]\{1,3\}\.\)\{3,3\}[0-9]\{1,3\} *$'
RE_NSV4OR6='^nameserver  *[a-fA-F0-9:\.]\{1,46\}\(%[a-zA-Z0-9]\{1,\}\)\{,1\} *$'

# Count how many IPv6 nameservers we can fit

limit=$RDNSS_LIMIT

nnsv4=`grep -c "$RE_NSV4" $resolvconf || [ $? -le 1 ]`
room=$(($MAXNS - $nnsv4))

if [ $limit -lt $room ]; then
	limit=$room
fi

# Merge and write the result. Let rdnssd assume ownership of all IPv6
# nameservers, and remove extraneous IPv6 entries as expired. However
# DHCPv4 most often sets up search list entries, and rdnssd cannot
# clobber these lest it causes counterintuitive breakage. There is no
# easy way to properly merge and manage DNSSL entries here, so just drop
# them.

{
	sed -e "/$RE_NSV4OR6/d" < $resolvconf
	grep -m $limit "$RE_NSV4OR6" < $myresolvconf || [ $? -le 1 ]
	sed -ne "/$RE_NSV4/p" < $resolvconf
} > $resolvconf.tmp

mv -f $resolvconf.tmp $resolvconf

